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Abstract

"During the Tertiary period, nuée ardente type eruptions spread large deposits of welded tuffs over southwestern Utah and southeastern Nevada. The Wah Wah Springs Tuff is a member of one of the most extensive of these units, the Needles Range Formation of Oligocene age. Its known outcrop area is estimated at 13,000 square miles. Relief was slight in the area during the time of eruption to enable the flows to spread over so large an area. In an effort to analyze the lateral and vertical continuity of the member, a petrographic study was undertaken. The average composition of the Wah Wah Springs Member was found to be 1.78% quartz, 25.9% andesing (An44.2), 4.0% biotite, 6.% hornblende, 0.3% diopside, 1.6% magnetite totaling 39.8% crystals, with no significant lateral variations throughout its studied extent. Vertical variations in mineral concentration were found. These variations are due to a time differential between successive eruptions forming a single cooling unit. The variations record changes occurring in the magma chamber or mixing of mamas before or during eruption. The density and relative porosity of the bulk rock samples were also analyzed. The vertical variations found in these parameters are indicators of the degree of welding. The changes in welding found in the Wah Wah Springs Tuff correspond to the changes described in much of the published literature. These changes are a moderately welded tuff at the base grading into a densely welded zone approximately one-third of the distance to the top of the member and gradual decrease in welding upwards, to a degree lower than that at the base"--Abstract, page ii.